You are visiting the web site of the EPSCI Program at the U.S. Dept. of Energy's Ames Laboratory, Ames, IA
On-site demonstration by contractor
Ames Laboratory fielded this technology within the ESC project at the Marshalltown, Iowa FMGP site. Geoprobe Systems, Inc. was the contractor performing the electrical conductivity logging.
Summary:
At the Marshalltown FMGP site, Geoprobe's Colin Christy and Phil McElhinney obtained 27 logs to depths of approximately 50 feet using the conductivity probe. The logs were then used to determine the confining bed of clay where contaminants could be found. These logs were merged nto 3D models and used to determine soild sampling depths and locations. McLaren/Hart from Sommerville, NJ, used their Geoprobe unit to collect large bore soil samples at the identified locations and confirmed the information in the conductivity logs.
A percussion soil probe system designed for direct sensing of soil conductivity was developed recently by Geoprobe Systems, Inc. The system was field tested in Marshalltown as a comparable technology to the friction ratio capabilities of the CPT system.
The probe is approximately 1-1/8 inches in diameter with isolated contacts. The probe is advanced into the ground by a hydraulically powered pushing and percussion mechanism. A signal cable attached to the probe is run through the inside of the rod to a laptop computer-based data recording system. Depth measurements are obtained from a stringpot system configured to measure the distance from the driving mechanism to the ground surface. Data derived from this system is used to both determine the depth of the probe and the probing speed. Conductivity measurements are displayed with respect to depth in near-real-time on the computer screen. Data was recorded in spreadsheet format and downloaded at the end of each field day.
High conductivities are associated with clayey soils, low conductivities with sandy and gravely soils, and sand/silt/clay mixtures have conductivities that lie somewhere in between. Conductivity levels change somewhat from one sample location to the next; however, the clayey soils at the site tend to have conductivities in the range of 60 to 140 mS/m and the sands about 30 to 40 mS/m, with gravels generally showing a conductivity dip of about 5 to 10 units below the level for a sandy soil. Because the conductivity varies somewhat for a given soil type between sample locations, comparing the conductivity log with a soil boring log or CPT soil classification log is essential to properly interpret the conductivity log. Changes in conductivity are caused by changes in soil type, water content, and soil or groundwater contamination. Accordingly, when used alone, the conductivity log leaves some doubt regarding an accurate soil classification. It is quite effective, however, at delineating changes in the subsurface properties.
Research by Geoprobe Systems, Inc. has shown the probe to produce reproducible logs and to provide a high degree of vertical resolution for the discrimination of soil/sediment units. The probe electrodes can be operated in either a Schlumberger or dipole array. The dipole array has the potential of providing a higher degree of vertical resolution due to the closer electrode spacing.
A total of 27 conductivity profiles were collected. Each push was halted upon a confident identification of the lower cohesive unit (i.e., rapid rise in soil conductivity) or in locations where the lower cohesive unit was absent, upon probe refusal.
In most conductivity logs, the top 3 to 5 feet of the section demonstrated erratic conductivity values. Possible explanations offered for this phenomenon were infiltration of highly saline solutions resulting from salting of road during winter weather and poor ground-to-probe contact at shallow depths. The upper cohesive unit/granular unit interface and the granular unit/lower cohesive unit interface could be easily identified in most locations of the site. Clay-rich layers within the granular unit could also be identified in several profiles and were confirmed by CPT friction ratio data or descriptive drill logs.
The conductivity logging system produced 700 feet of log in 27 holes over a period of 5 working days. The system was operated by a two-man crew. Operation by a one-man crew is possible, although productivity would be significantly lower. The data required minimal post-processing (deletion of negative or repeat values). Digital conductivity and probing speed data and field printouts were provided at the end of each work day for integration into the existing site model.
Occasional problems encountered during the completion of this work included electrode failure, probe point mechanical failure, intermittent negative values in conductivity data, erratic or extremely high conductivity values in the upper few feet of most profiles.
The advantage of this system is its ability to provide a large number of profiles in a relatively short time and it can be operated by a single person. It is also rather versatile in that it can maneuver into small spaces and can penetrate most unconsolidated subsurface materials.
A short-coming of the system was its inability to easily identify weathered carbonate bedrock, which was encountered in some locations where the lower cohesive unit was absent. Weathered bedrock displayed erratic conductivity values not easily always distinguishable from inhomogeneity in the overlying units. Positive identification of the material could only be made subsequently with core samples.
Movie
References:
"A Percussion Probing Tool for the Direct Sensing of Soil Conductivity," presented at the 8th Annual Outdoor Action Conference, May 1994.
For more information contact:
Colin Christy, Geoprobe Systems, (913) 825-1842.
Please e-mail comments to: epsciwebkeeper@ameslab.gov.
Last Modified: 1 January 2002 by dave eckels
Expedited Site Characterization: etd/technologies/projects/esc/technologies/eleccond.demo.html